1. Field of the Invention
The present invention relates to hydraulically damped mounting devices.
In a structure where it is desired to prevent the vibration of a piece of machinery (reciprocating engine for example) being transmitted to another part, it is common to provide an isolator (or mounting) between the two parts of the structure, the isolator being a combined spring and damper. The present invention is concerned with a mounting device where the damping is achieved at least partially hydraulically. It finds applications, for example, in mounting an engine in a vehicle. In general such mountings have to function under different types of vibrations, which may vary both in frequency and amplitude. In general, small amplitude vibrations have a high frequency and large amplitude vibrations have a low frequency, but this is not always the case due to e.g. resonances in the system. The resonances may arise both from the structure itself and from properties of the mounting device.
2. Description of the Prior Art
Conventional mountings usually have and all-elastomeric construction to provide a compromise between the requirement of high damping to control and limit large amplitudes at predominantly low frequency and the requirement at low damping to reduce the forces transmitted at small amplitude and high frequency. The present invention overcomes the need to compromise by providing two stage damping, i.e. to achieve a large degree of damping for high amplitude vibration but negligible damping at low amplitudes.
In UK No. 811748 a mounting was proposed which attempted to achieve this aim. It consisted of a first anchor point to be attached to one part of the structure, a second anchor point to be attached to another part of the structure, and a deformable wall in the form of a resilient spring element connecting together the fixing points. A surface of a flexible partition rigidly associated with the first anchor point defined, together with the spring element, a working chamber containing liquid. The working chamber communicated, via an aperture in the flexible partition with a compensation chamber or reservoir. The compensation chamber was bounded by a second deformable wall in the form of a bellows so that the volume of the compensation chamber was determined by the liquid pressure in it, which in turn was determined by the volume of the working chamber. Vibrations of one part of the structure relative to the other caused the anchor points to vibrate relative to each other and this caused flexing of the spring element thereby changing the volume of the working chamber. For small amplitude vibrations, flexing of the diaphragm absorbed the movement of hydraulic fluid caused by the change in the volume of the working chamber. For high amplitude oscillation, however, this movement was insufficient to take up all the volume change and hydraulic fluid moved between the working chamber and the compensation chamber through the aperture, the compensation chamber expanding or contracting by deformation of the second deformable wall to compensate for the movement of fluid. The fact that part of the second deformable wall was free to move is important to ensure that the compensation chamber acts as such. The volume of the compensation chamber must be variable in such a way as to respond to changes of pressure within the compensation chamber. Thus, the structure damped large oscillations but left small oscillations relatively undamped.
Numerous attempts have been made to improve the mounting of No. GB-A-811748. It is known from No. EP-A-0027751 and No. EP-A-0040290 to form the partition as a flexible membrane in a rigid support fixed to the first anchor point. Instead of an aperture in the flexible membrane, a long passage of small cross section but of significant volume is provided in the rigid support connecting the working and compensation chambers. The flexible membrane is to absorb low amplitude vibrations, namely those whose amplitude is not great enough to disturb the column of liquid in the long passage. Both European publications are concerned with detailed proposals for the construction or dimensions of the passage with a view to controlling the response of the assembly to high amplitude vibrations.
Neither of these publications, nor No. GB-A-2068079 (which also presents the idea of a differentiated response to high amplitude vibrations on the one hand and low amplitude vibrations on the other, but in a different type of mounting device) has any concept of a specifically frequency-responsive device.